Graphene for sensor applications has received considerable attention due to the material's unique physicochemical properties, such as large surface to volume ratio, high mechanical strength, biocompatibility, excellent thermal and electrical conductivity, low cost, safety and ease of production. Graphene also exhibits a broad electrochemical potential and low charge-transfer resistance, making it almost ideal for multifunctional fast sensors. Another important feature required in optical biosensors is the ability to be functionalized. Both graphene and graphene oxide (GO) are versatile materials for functionalization.
This multitude of favorable properties has led to a wide array of investigations into graphene use for biosensing. Particularly interesting configurations are graphene field-effect transistors (GFETs) and graphene enhanced surface plasmon resonance (SPR). These types of graphene sensors have been used for DNA, protein, glucose, and bacteria detection.
Graphene field effect transistors (GFET)
GFET is a modification of the classic silicon field-effect transistor, ubiquitous in modern electronics. In traditional transistors, silicon acts as a thin conducting channel, the conductivity of which can be tuned with applied voltage. GFETs perform in a similar manner, except that the silicon is replaced with graphene, which yields a much thinner and hence more sensitive channel region. Due to the broad electrochemical potential and ability to be functionalized, GFETs present an attractive device for biomolecules to attach to, and because of graphene's ultimate thinness and extreme surface-to-volume ratio, even the smallest concentration of attached molecules changes the channel conductivity. GFET biosensors are available for sensing enzymes, hydrogen peroxides, dopamine, and reduced b-nicotinamide adenine dinucleotide (NADH) molecules.
Surface plasmon polariton detectors
In a different configuration, graphene or GO is used in conjunction with surface plasmon polaritons (SPPs) on metal films to enhance biosensor performance. SPP-based sensors utilize the confinement of optical waves on the surface of metals to make small-volume chemical and biological sensors. The sensing volume is given by the tightly confined surface wave, boosting sensitivity of optical detection. Standard metals used in this type of sensor are gold and silver, due to their favorable SPP propagation properties. However, silver corrodes quickly, and gold has poor adsorption properties. Placing a layer of graphene on top of the gold results in superior adsorption, and GO is in particular good at binding proteins, due to its high covalent binding affinity. GO multiplies the sensitivity of SPP sensors in this label-free detection system. In conjunction with microfluidics technology, researchers have shown that they can detect a single cancer cell. In a most recent result, scientists announced that GO-based chips could be used for detection of HIV. The selectivity of the GO SPP sensor was enhanced by adding streptavidin to the GO coating. Adding selectivity to GO SPP sensors has been a rising trend in research, with several recent reports on the topic. Selectivity can also be improved by utilizing several graphene measurement modes at once, such as mechanical, electrical, and optical.
Figure: SPP sensor enhanced with graphene (Reproduced from Optics Express 18, 14395 (2010) with permission from the Optical Society of America).
Aside from GFETs and SPPs, several other types of graphene-enhanced sensors are on the rise, such as for example micromechanical (MEMS) and pressure sensors.